1. Field of the Invention
The present invention relates to stepping motors and, more particularly, it relates to a stepping motor especially suitable for positioning a read/write head of a disk storage device.
2. Description of the Related Art
Stepping motors are commonly used for accurately positioning various components, such as the read/write head of a disk storage device, so as to move the component through a desired distance and then stop the driven component at a desired precise position. To this end, designers of stepping motors have searched for ways in which to improve the velocity and accuracy of stepping motor operation.
Stepping motors are made with a multi-phase winding structure, usually a two phase winding, so that the rotor thereof is rotated in step increments of a small angle whenever a current vector supplied to the multi-phase coil is switched. Theoretically, the stepping motor velocity can be increased with increased current vector switching and positioning accuracy can be increased with reduction in the rotational angle of each stepping increment. Both stepping motor speed and positioning accuracy, however, require that the inertia of the driven component be matched with the torque developed by the stepping motor. For example, in the operation of a read/write head of a disk storage device, a mechanical coupling between the head and the stepping motor is kept as light as possible so that the head positioning operation can be performed by a small, low inertia stepping motor to move the head through the required maximum distance relative to the disk surface in approximately 40-50 milliseconds. The positional area of the head falls within the range of from 2-3 .mu.m.
If it is attempted to shorten the head moving time to about a period on the order of 20 milliseconds, for example, by increasing the operating velocity of the stepping motor, the actual time required to move and stop the head at a target position cannot be reduced with further reduction in mass of the mechanical coupling system alone. This is because the head oscillates before and after a target position is reached, thus requiring time for the head to become stopped in a stable condition. The oscillation is caused by fluctuations in the rotating velocity of the stepping motor as may be appreciated from FIGS. 5 and 6 of the accompanying drawings.
In FIG. 5, a theoretical schedule of the velocity v relative to the minimum time t required for moving a read/write head by a predetermined distance is designated Vs. As shown, the velocity schedule Vs is trapezoidal in shape so that the head is accelerated with the full torque of the stepping motor from a time t0 to a time t1. The head maintains a constant velocity from the time t1 to the time t2. Finally, the head is decelerated with full torque of the stepping motor from the time t2 to the time t3.
In the period of time from the time t1 to the time t2, however, a wave is generated in the curve Va representing actual velocity of the head in FIG. 5. Although the acceleration and deceleration represented by the wave is not wanted during this period of time, such variations in velocity are the result of rotational fluctuations in the stepping motor caused by switching the phase current vector. The influence of the rotational fluctuations thus appear as fluctuations in an intended constant velocity. Accordingly, the actual velocity Va of the head becomes different from the scheduled Vs at the time t2 from which the head is to be decelerated. For example, if the actual velocity Va is high at the time t2, as shown in the drawing, deceleration of the head is correspondingly delayed.
FIG. 6 is a graph in which the moving distance x of the head is related to time t. If the deceleration is delayed as described above, the actual moving distance x of the head, up to the time t3, at the end of the scheduled deceleration period, becomes larger than the scheduled distance xs. This means that the head passes through the target position existing at the scheduled distance xs. That is, after the time t3, the head oscillates before and after the target position as shown in FIG. 6 and stops at the target position at a time t4 which is considerably delayed.
Although it is assumed that the actual velocity Va is higher than the scheduled velocity Vs at the time t2 when deceleration is initiated in the example of FIG. 5, the actual velocity Va could be lower than the scheduled velocity Vs. Although in this latter case, deceleration begins sooner, oscillations generated after the time t3 cause the driven component to move before and after the target position, giving rise to the same delay represented in FIG. 6.
As may be appreciated from the above description, if the inertia of the driven component is reduced to increase operating velocity, the influence of the rotational fluctuations of the stepping motor have a greater effect on velocity variations of the driven component. Accordingly, increasing the operating velocity of the stepping motor is of limited effect in shortening the actual operating time required to move and position the driven component.
In light of the limitations on the speed of positioning movement using a stepping motor, the problem has been addressed by attaching a damper to the stepping motor so as to absorb the rotational fluctuations of the motor. An example of a stepper motor including such a damper is shown in FIG. 7 of the drawings.
In FIG. 7, a stepping motor 10 is shown in which a stator, having magnetic poles 2 and coils 3 wound on the magnetic poles 2, is housed in a casing 1. A rotor, including a pair of iron rotor cores 6 and a magnetic-flux generating permanent magnet 7, is mounted on a shaft 5 supported by the casing 1 through bearings 4. Provided under the stepping motor 10, is a oil damper 30 in which a plate-like metal ring 32 and high viscosity oil 33 are sealed in a damper casing 31 which is attached on the lower end of the shaft 5. If the rotating velocity of the stepping motor 10 changes suddenly, a difference in velocity is produced between the damper casing 31 and the metal ring 32, so that the change of the velocity is damped by the viscosity of the oil 33 regardless of whether the velocity change is acceleration or deceleration.
The damper 30 described above is a so-called inertia viscosity type damper effectively using the inertia of the metal ring 32 and the viscosity of the coil 33, and develops a high damping effect on sudden changes of the rotating velocity of the stepping motor. However, the damper 30 has the following problems.
(a) The inertial load on the motor is increased considerably by the damper thereby offsetting increases in operating velocity. In order to compensate for the increase in inertia, it is necessary to increase the torque of the stepping motor. Accordingly, where it is desired to make the operating velocity high, the additional provision of the damper is likely to require an increase in the physical size of the motor.
(b) As may be seen from FIG. 5, the suppression of rotational fluctuations in the stepping motor through damping is effective during the period of high but constant velocity. It is, however, desirable to inhibit damping during low-velocity rotation. The damper is effective against any change in velocity in view of the operational principals thereof. Accordingly, operation velocity is apt to be reduced by the damper, particularly in the case where the distance of driven component movement is short.
(c) The additional provision of the damper increases the physical size of the stepping motor. Where the stepping motor is used for driving a head of a disk storage device, any increase in the axial size of the stepping motor is particularly disadvantageous.
The latter problem may be appreciated by reference to FIG. 8. In a thin disk storage device, the stepping motor 10 is attached to a casing 40 at a recessed portion thereof so as to be flush with a base plate 50. The thickness of the overall drive is thus kept to approximately one inch. The thickness of the stepping motor 10 is 1/2 inch at the minimum. It is necessary, however, to house the mechanism (not shown) for coupling the stepping motor 10 to a head, for example, within the casing 40. Therefore, it is also difficult to further increase the depth of the recessed portion of the casing 40. Hence, if the damper is additionally mounted on the motor, the axial dimension of the overall drive is increased to accommodate the damper.